Daniel Moye's Technical Writings

SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES

Origin times obtained in this manner from each station are compared; gross
discrepancies may require re-interpretation of the records. Epicentral distances
are calculated using the adopted origin time and an assumed depth, and
corresponding arcs are drawn from each station on a map of the area. The
epicentre will be located at the intersection of the arcs: the coincidence of arcs
thus provides a further check on the validity of the interpretation.

In practice, it is found that shocks in the area which are large enough to
be recorded at all stations can generally be interpreted without difficulty.
Occasionally a station in close proximity to the epicentre may record a “false S”
(Richter, 1958, p. 293), which can be readily identified by the above technique.

When first approximations to origin time, latitude, longitude and focal
depth have been calculated as above, these are fed into the computer together
with weighted arrival times, station co-ordinates, and wave velocities. Using
a least-squares iterative process, optimum values of the parameters are derived,
with standard errors.

The computed epicentres, focal depths and origin times, with standard
errors, are listed in Table 1. The epicentres have been mapped in Fig. 1;
standard errors in latitude and longitude are indicated by rectangles about the
epicentres. The high accuracy of the determinations was made possible by
the close proximity of the network to the shocks. In a few cases the programme
did not give a convergent solution for depth of focus, although the other
parameters were well determined; in the figure these epicentres are represented
by crosses. The projected section shows the depths of foci, with standard
errors, for the relatively active region between Wambrook and Jindabyne. It
may be noted that the majority of these foci are at depths close to 10 km.

In the eight months prior to the Berridale earthquake only three movements
in the Snowy Mountains were recorded, none of which were in the
Berridale region. In the fifteen months following the event, at least 25 minor
shocks occurred, which were confined almost exclusively to an area within 20
km. of the Berridale epicentre. Activity subsequently diminished in that area,
but at the same time movements became more numerous in other parts of the
Snowy Mountains. Although the Berridale tremors probably represent an
aftershock sequence, their behaviour is atypical in that no activity was observed
in the few days following the major shock. This is in contrast to the Robertson-Bowral
sequence, in which about 80 aftershocks were recorded within three days of the earthquake
(Cleary and Doyle, 1962).